The present invention relates to a .lambda./2 wavelength mode ultrasonic beam focusing device with a concaved piezoelectric polymer film, and to a method of manufacturing the same.
The use of oscillation in the direction of thickness of a piezoelectric polymer film in an ultrasonic beam focusing device for ultrasonic diagnosis has recently been studied. In such studies, the thickness of a piezoelectric polymer film is determined by the frequency of a transmitted or received ultrasound or ultrasonic beam and the mode of oscillation of the film. However, in general, since the frequency of ultrasonic beams for ultrasonic diagnosis is from several MHz to several tens of MHz, the thickness of the piezoelectric polymer film may be within the range of about 30 to several hundred micrometers. However, when a piezoelectric polymer film has a thickness falling within this range, the film as a piezoelectric oscillator cannot retain its shape. In view of this problem, a .lambda./2 wavelength mode ultrasonic transducer has been proposed which has a piezoelectric polymer film adhered to some type of a support, as shown in FIG. 1. More specifically, electrodes 2a and 2b are formed on the two major surfaces of a piezoelectric polymer film 1. The electrode 2a is adhered to a support 3 through an adhesive or the like, while the electrode 2b is adhered to a matching layer or an electrically insulating layer 4. A pair of lead wires 5 are respectively connected to the electrodes 2a and 2b. The matching layer 4 effectively propagates an ultrasonic beam received or emitted by the piezoelectric polymer film 1. The matching layer 4 also electrically insulates the electrode 2b from an object to be examined. The support 3 must stably hold the piezoelectric polymer film 1 and must not reflect the ultrasonic beam received by the piezoelectric polymer film 1 in any direction other than toward the object. The support 3 must also have wide-band characteristics, a good response and a small conversion loss.
In view of this situation, Japanese Patent Laid-Open Publication No. 55-163999 (piezoelectric polymer transducers) proposes a .lambda./4 wavelength mode ultrasonic transducer with a foamed sheet which has an acoustic impedance smaller than that of a piezoelectric polymer film and which has more small pores. The foamed sheet, in this context, means a sheet of foamed styrol, foamed polyethylene or foamed polyurethane; or a sheet comprising a film of a polymer, a metal, ceramics, glass or the like which has a number of small pores or concavities formed by chemical etching, machining or electric-discharge machining. However, since such a foamed sheet is included as an additional layer to the piezoelectric polymer film, it must be adhered to a support of an acrylic or epoxy resin with an adhesive. This presents the difficulty of controlling the film thickness of the additional layer and of loss of the ultrasonic beam through the assembly of the additional layer and the support.
Meanwhile, in the field of ultrasonic beam focusing devices, it is known to concave a piezoelectric polymer film so as to focus the ultrasound beam emitted from the film at a single point in an acoustic propagation medium or in an object to be examined, in order to generate an intense ultrasound field and thus to improve resolution of the focusing device. This technique is disclosed in Japanese Patent Laid-Open Publication No. 53-25389 entitled "Ultrasound beam focusing device". In this prior art technique, a piezoelectric polymer film is adhered to a concaved electrode or an electrode supported on a concaved support. Alternatively, electrodes are adhered to two major surfaces of a piezoelectric polymer film and the overall assembly is pressed and made concave. However, with this method, the precision in the radius of curvature of the concave portion may be low. Additionally, the electrodes may not be sufficiently adhered to the piezoelectric polymer film, thus resulting in separation of the film.